Cell Life Cycle
• Cells have two major periods– Interphase• Cell grows• Cell carries on metabolic processes
– Cell division • Cell replicates itself• Function is to produce more cells for growth and repair
processes
DNA Replication
• Genetic material is duplicated and readies a cell for division into two cells
• Occurs toward the end of interphase• DNA uncoils and each side serves as a
template
DNA Replication
Figure 3.14
Events of Cell Division
• Mitosis—division of the nucleus– Results in the formation of two daughter nuclei
• Cytokinesis—division of the cytoplasm– Begins when mitosis is near completion– Results in the formation of two daughter cells
Stages of Mitosis
• Prophase– First part of cell division– Centrioles migrate to the poles to direct assembly
of mitotic spindle fibers– DNA appears as double-stranded chromosomes– Nuclear envelope breaks down and disappears
Stages of Mitosis
• Metaphase– Chromosomes are aligned in the center of the cell
on the metaphase plate
Stages of Mitosis• Anaphase– Chromosomes are pulled apart and toward the
opposite ends of the cell– Cell begins to elongate
Stages of Mitosis
• Telophase– Chromosomes uncoil to become chromatin– Nuclear envelope reforms around chromatin – Spindles break down and disappear
Stages of Mitosis
• Cytokinesis– Begins during late anaphase and completes during
telophase– A cleavage furrow forms to pinch the cells into
two parts
Stages of Mitosis
Figure 3.15, step 3
Centrioles
Plasmamembrane
Interphase Early prophase Late prophaseNucleolus
Nuclearenvelope
Spindlepole
ChromatinCentrioles
Formingmitoticspindle
Centromere
Chromosome,consisting of twosister chromatids
Fragments ofnuclear envelope
CentromereSpindlemicrotubules
Stages of Mitosis
Figure 3.15, step 6
Metaphase Anaphase Telophase and cytokinesis
Daughterchromosomes
Sisterchromatids
Nuclearenvelopeforming
Nucleolusforming
Spindle Metaphaseplate
Cleavagefurrow
Protein Synthesis
• Gene—DNA segment that carries a blueprint for building one protein
• Proteins have many functions– Building materials for cells– Act as enzymes (biological catalysts)
• RNA is essential for protein synthesis
Role of RNA
• Transfer RNA (tRNA)– Transfers appropriate amino acids to the ribosome
for building the protein• Ribosomal RNA (rRNA)– Helps form the ribosomes where proteins are built
• Messenger RNA (mRNA)– Carries the instructions for building a protein from
the nucleus to the ribosome
Transcription and Translation
• Transcription– Transfer of information from DNA’s base sequence
to the complimentary base sequence of mRNA– Three-base sequences on mRNA are called codons
Transcription and Translation
• Translation– Base sequence of nucleic acid is translated to an
amino acid sequence– Amino acids are the building blocks of proteins
Protein Synthesis
Figure 3.16
Nucleus(site of transcription)
DNA
mRNA specifyingone polypeptideis made onDNA template
mRNA leavesnucleus andattaches toribosome, andtranslationbegins
Synthetaseenzyme
Amino acids
Cytoplasm(site of translation)
Correct aminoacid attachedto each speciesof tRNA by anenzyme
Growing polypeptide chain
Nuclear pore
Nuclear membrane
mRNA
As the ribosomemoves along themRNA, a new aminoacid is added tothe growing proteinchain
Released tRNAreenters thecytoplasmicpool, ready tobe rechargedwith a newamino acid
Direction of ribosomeadvance; ribosome movesthe mRNA strand alongsequentially as each codonis read
Small ribosomalsubunit
Portion ofmRNA alreadytranslated
tRNA “head” bearinganticodon
Large ribosomal subunit
Peptide bond
Incoming tRNArecognizes acomplementarymRNA codon callingfor its amino acid bybinding via itsanticodon to thecodon
Codon
AlaPhe
Ser
GlyMet
C G G
GU UU C UCC AA G CCA U
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